That's your error bar. When you press a key to hit a note, that bar will help tell you how precise you were.
The white center line indicates "perfect timing", so that's 0ms off the actual note's placement. Anything to the left of it indicates that you pressed it a little early, and anything to the right of it indicates that you pressed it a little late.
If you're in the blue, then awesome! That's within the timing window of a 300.
If you're in the green, watch out! You're within the timing window of a 100.
If you're orange, ouch. That's a 50.
The hit error bar is incredibly important in learning why you hit inaccurately. If you want to improve your accuracy, you definitely want to pay attention to it, at least during replays.